Concrete prestressing apparatus



Aug. 13, 1968 J. w. HOWLETT CONCRETE PRESTRESSING APPARATUS (JACK) 2 Sheets-Sheefl Original Filed Aug. 16, 1963 INVENTOR.

James W Howler! Aug. 13, 1968 Original Filed Aug. 16, 1963 J. w. HOWLETT 3,396,943

CONCRETE PRESTRESSING APPARATUS (JACK) 2 Sheets-Sheet 2 INVENTOR. James W Howler! United States Patent CONCRETE PRESTRESSI G APPARATUS (JACK) James W. Howlett, Richmond Annex, Calif., assignor to Howlett Machine Works, a corporation of California Original application Aug. 16, 1963, Ser. No. 302,628, now

Patent No. 3,343,808, dated Sept. 26, 1967. Divided and this application Sept. 20, 1967, Ser. No. 669,121

3 Claims. (Cl. 254-77) ABSTRACT OF THE DISCLOSURE The present application is a division of U8. patent application S.N. 302,628, filed on Aug. 16, 1963, now Patent No. 3,343,808, for Concrete Prestressing Apparatus.

The invention relates generally to the art of prestressing various concrete structural members by the use of tensioning tendons such as wires, rods, stranded cables or bars, and more particularly to an apparatus for drawing together two generally aligned end opposed tendons especially suited for use in conjunction with the method and apparatus described in my copending patent application S.N. 302,628, filed Aug. 16, 1963, for Concrete Prestressing Apparatus, of which the present application is a division.

In various concrete prestressing operations, it is required that tension be applied to elongated tensioning tendons. This may involve either pretensioning or posttensioning of such tendons in relation to the concrete to be prestressed. Some tensioning techniques involve securing a rod at one of its ends to a stationary object, and then pulling the rod from its other end and securing it with the rod under tension. In reinforcing large cylindrical tanks or the like, it has been common to wind wire or rods around the outer sidewall of the tank as a form of post-tensioning, which creates the desired radially inward prestressing forces. A disadvantage of the wire winding technique is that the Wires are relatively small, and therefore are subject to corrosion and breakage whereby maintenance becomes a significant problem. The use of steel rods in prestressing tanks has generally involved two end opposed axially aligned rods secured at their remote ends, with means being provided for drawing together the adjacent ends of the rods to place them under tension. Heretofore this has been accomplished by the use of turnbuckles or similar devices involving threaded couplings. Aside from the tedious task of tightening a turnbuckle, this type of tensioning device is not capable of achieving the same high order of prestressing that can be withstood by modern special alloy steel rods. For example, rods are available that have a tension strength in excess of 100,000 pounds per square inch, and which can withstand tension forces much greater than that provided in practice by turnbuckle devices. Moreover, it has been found impractical to use coupling devices wherein threads are formed in the tensioned rods, inasmuch as the threads create weakened shear planes incapable of withstanding high tensile forces of the magnitude above mentioned. Another technique has been to provide heads or enlargements on the confronting ends of the tendons. These heads are then engaged by plates which may be drawn together by a hydraulic jack, after which bolts are mounted through the plates in connecting fashion and nuts run down on the bolts to hold the plates against separation upon subsequent release of the hydraulic jack. The plates and bolts thus form a holding frame or bracket for the tendons. While effective as a means for holding tendons under stress, the foregoing arrangement is relatively large and bulky, and considerably time consuming. Also, Where center stressing of tendons is used in prestressed floor slabs, the foregoing arrangement may be too large and bulky for convenient use.

Briefly, the present invention including means described in S.N. 302,628 for engaging two adjacent ends of opposed tendons, is characterized by a unique jack means for drawing the tendons together under tension.

Accordingly, it is a main object of the present invention to provide an improved method and apparatus for tensioning rods by drawing together and connecting them, and whereby the rods can be placed under large tensile forces approaching their maximum tensile strength.

Another object is to provide a method and apparatus of the character described for quickly, simply and efiiciently connecting two spaced end opposed rods and placing them under high tensile stress.

A further object is to provide a method and apparatus of the character described capable of prestressing circular tanks or the like by applying tension to rods disposed circumferentially around the tanks outer wall.

It is a more specific object of the invention to provide a jack apparatus which can be simply and quickly attached across opposed rod tendons for drawing same together under unusually high tension so that wedge grip couplers of the type described in S.N. 302,628 can secure the rods ends under such tension after relaxation of the jack apparatus.

The invention possesses other objects and features of advantage, some of which of the foregoing will be set forth in the following description of the preferred form of the invention which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by the said drawings and description may be adopted within the scope of the invention as set forth in the claims.

Referring to said drawings (two sheets):

FIGURE 1 is a side elevation of a concrete tank depicting also the method and apparatus of the present invention for post-tensioning encircling rods employed to prestress the tank.

FIGURE 2 is a plan view, in an enlarged scale and partially cut away and in cross-section, of tensioning apparatus embodying the present invention.

FIGURE 3 is a fragmentary side view of a portion of the apparatus as suggested by the line 3-3 in FIG- URE 2.

FIGURE 4 is a cross-sectional view taken substantially on the plane of line 4-4 of FIGURE 2.

FIGURE 5 is a cross-sectional view along plane 55 of FIGURE 3 of a one-way wedge grip coupler forming a part of the present invention, and operatively engaging two end opposed bar portions.

FIGURE 6 is a fragmentary plan view partially cut away and in cross-section, similar to FIGURE 2, but depicting tensioning apparatus forming an alternative embodiment of the invention.

Referring now to the drawings, the apparatus of the present invention provides means for drawing together and tensioning two generally aligned tendons 11 and 12 having opposed spaced ends 13 and 14, and includes a bar member 16; at least one coupler member 17 having gripping means 18 for axially receiving the bar member 16 and holding it against return movement; one of the members 16 and 17 being mounted on the tendon 11 in opposed confronting relation to the tendon 12; means for mounting the other of said members 16 and 17 on the tendon 11 in position for operative engagement with the member on the tendon 12; jack engaging means 19 and 21 provided respectively on each said tendon 11 and 12;

and jacking means including jaws 23 and 24 for engaging the jack engaging means 19 and 21 for forcing-the tendons 11 and 12 towards one another and operatively connecting the members 16 and 17 together, whereby the jacking means can be released to leave the tendons and members under tension.

In the embodiment of the invention depicted in FIG- I URES 1-5, and in particular FIGURE 2, the means for mounting the bar 16 on the tendon 11 includes a second coupler 26 mounted on the tendon 11 and adapted to ,receive the end 27 of the bar member 16 in the same manner as does the coupler, 17. The remote end shoulders of the couplers 17 and 26 are seen in this embodiment to provide the aforesaid jack engaging means 19 and 21, and the jaws 23 and 24 engage these shoulders to draw the tendons together.

As more specifically regards the structure of the couplers 17 and 26, a single one of which is shown in FIG- URE 5, reference is made to patent application S,.N.302,-, 628, wherein the couplers are, described in detail. For the present, however, couplers 17 and 26 may be considered as gripping wedges in which each coupler may receive a pair of rod ends such as tendon 11 and bar member 16 through opposing ends and secure them against axial tension.

The method of the present invention in broad terms consists in the steps of mounting the coupler member 17 on the tendon 12 and providing the bar member16 on the end of the tendon 11, which in the embodiment depicted in FIGURE Zinvolves the provision of a coupler 26 secured to the tendon 11 and adapted to grippingly receive the end 27 of the bar 16. Force is applied by the jacking means to urge the tendons together and to bring 1 the couplers 17 and 26 into gripping connecting engagement with the bar 16. In the embodiment depicted in FIGURE 2, the bar 16 is initially placed between the couplers 17 and 26 in spaced relation therewith, after which the couplers are drawn together into engagement with the respective ends of the bar. The force of the jacking means is then slowly released thereby enabling the bar to be gripped securely by the coupler wedges. It is noted that the tension initially provided by the jacking means is r diminished after the force is released and as the teethof the wedges bite into the bar. More specifically, as the teeth bite radially into the bar, there is a longitudinal displacement of the wedge relative to the coupler bore, which occurs primarily from the relative movement of the aforesaid interfitting, cam planes of the wedge andhousing bore.

In order to achieve a maximum tensionin the rods, it is frequently desirable to remove the slack created by the aforesaid setting of the wedges. In the embodiment depicted in FIGURE 2 this is accomplishedby; applying force to the couplers by the jacking means a second time, whereupon the tensile forcesin the bar 16 are subgagement with the connecting bar 16 as previously described. The fluid pressure means 49, as best seen in FIG- URE 2, is of generally conventional construction and consists of a' housing providing an interior cylinder 51 within which is mounted for reciprocation a piston 52. One end 53 of the housing is attached by a bracket 54 to arm 41, while the piston 52 is connected to the other arm 42 by a rod 55 which here projects axially through the housing 50 and piston 52 so as to project from a piston shaft section 56 which extends through the opposite end 57 of the cylinder housing. A wedge grip nut 58, to be described more fully hereinafter, forms a protuberance on the outer end 59 of the rod 55 so as to bear against the adjacent end of piston shaft section 56. The opposite end 6t) of the rod 55 may be similarly connected to the arm 42 by a wedge grip nut 61 which is arranged to bear against a flange 62 provided on a mounting bracket 63 ,fixed to the arm 42(Preferably, the mounting brackets 54 and 63 are of U-shape and are pivotally secured to arms 41 and 42 as illustrated. Housing 50 is here provided with inlet and outlet fittings 64 and 65 located adjacent the opposite ends 53' and 57 of the cylinder for conveying fluid under pressure into and out of the cylinder/As will be observed, the introduction of fluid under pressure into inlet fitting 64 will cause a displacement of piston 52 to the left, as seen in FIGURE 2, thus effecting a closing of the arms 41 and 42 and jaws 23 and 24. Contrariwise, the admission of fluid into the cylinder by way 01": fitting 65 and the withdrawal of fluid by way of fitting 64 will permit a displacement of piston 52 to the right, as seen in the drawing, and an opening of the arms and jaws. It will be noted that the rod 55 is disposed intermediate the link 43 and the free ends of the arms 41 and 42, and is substantially parallel to the link 43. The rod 55 is seen to be movable relative to the arm 41.

In further describing the arms and jaws of the jacking means, it will be appreciated that the two arms are symmetrical and substantially identical to one another. Thus, reference is made now only to the arm 41 and jaw 23 mounted thereon. In particular, it is preferred that the free end 44 of the arm 41 includes a pair of bifurcated elements 44a and 44b adapted to fit around the corresponding tendon 11, as is shown in detail in FIGURES 3 and 4. Similarly, the jaw 23 also includes a pair of hifurcated elements 23a and 23b which fit around the tendon 11, and are in general alignment with the elements 44a and 44b. As shown, the elements 23a and 23b are spaced closer together than the elements 44:: and 44b,

, arm. In order to assure a firm grip of the couplers by the stantially eliminated, leaving it in an unstressed slack condition. By substantially taking up the slack in, the bar 16, the force of the jacking means again. canbe released to leave the rods at the desired tension. This procedure is described in S.N. 302,628.

With more detail now regarding the aforesaid jacking means of the present invention, there are provided a pair of arms 41 and 42 mounted on a link member 43 for pivotal movement relative to one another and having free 1 ends 44 and 46. The aforesaid jaws 23 and-24 are mounted respectively on the ends 44 and 46, and each have bearing surfaces 47 and 48 for engaging the corresponding couplers 17 and 26 in the apparatus of FIGURE 2. Fluid pressure means 49 is connected to draw the arms together whereby the couplers 17 and.26 can be moved into enjaws, each jaw is pivotally mounted on its respective arm whereby the jaw and coupler need not move relative to one another as the arms are drawn together. Furthermore, in order to withstand the high forces involved in tensioning rods with the instant apparatus, the jaw 23 and free end 44 respectively have complementary arcuate bearing surfaces 66 and 67. In this manner, pivotal movement of the jaw relative to the arm is allowed by while still providing a suflicient bearing surface between these two members to transmit high force from the arms through the jaws to the couplers and rods. Preferably, the surface 66 is convex and is complementary to the concave surface 67. A pair of strap-like members 68 and 69, secured rigidly to the elements 44a and 44b, pivotally support the jaw 23 by means of stub shafts 70 and 71, but because of the arcuate bearing surfaces these stub shafts need not support the full load transmitted to the tensioned rods.

As further regards the structure of the arms 41 and42, it is preferred that these arms consist respectively of pairs of spaced arm members 72, 73 and 74, 76. Considering the arm 41, the spaced bifurcated elements 44a and 44b may be formed as integral extensions respectively of the members 72 and 73. The arm members are secured rigidly together by means of nuts 77, bolts 78, and spacers 79.

The bar 55 extends through the spaces between the arm members, and similarly, the link 43 is pivotally secured between the arm members.

It will be seen that the arms 41 and 42 as shown mounted at spaced positions on the link 43 have their center lines in relatively parallel relationship. Movement of the arms towards one another thus requires a minimum of pivotal movement of the jaws 23 and 24 relative to their resepective arms, thereby minimizing the friction involved in moving the arms together. Inasmuch as the tendons 1'1 and 12 to be joined are not always spaced the same distance apart, it is desirable that the length of the link 43 be adjustable to provide the aforesaid generally parallel relationship of the arms. As here shown, link 43 is composed of a tubular center section 80 and a pair of removable end members 81 and 82 which are in turn pivotally connected to the adjacent ends of arms 41 and 42 by pivot bolts 83 and 84. Since the force on the link member during operation of the device is in compression, end members 81 and 82 are provided with flanges, as seen in FIGURE 2, for abutting the opposite ends of the tubular center section 80. Preferably, ring clamps 85 and 86 are mounted around the opposite ends of the tubular center section for squeezing down upon the inserted portions of end members 81 and 82 so as to hold the latter in place, the opposite ends of the tubular center section being slitted to permit compression by the ring clamps against the end members. Adjustment of the length of the link 43 may accordingly be conveniently effected by dismantling the center section 80 and end members 81 and 82 and replacing the center section with one of appropriate (longer or shorter) length.

An alternative jacking arrangement suitable for providing the desired parallelism of the jaws can be provided in lieu of the pivotable jaw arrangement just described. More specifically, with nonpivotable jaws formed on the free ends of the jack arms, there can be provided in place of the link 43 an adjustable link consisting of a conventional hydraulic piston and cylinder such as that included in the fluid pressure means 49 already described. In using such a link, force could be applied to the jaws to draw them together as described hereinabove, but in short increments, with the adjustable link being correspondingly shortened after each increment to maintain the arms in the desired parallel relation. The shortening of the link is accomplished in a simple manner by valve means for transferring fluid from one side of the piston to the other as is necessary.

An alternative embodiment of the invention is depicted in FIGURE 6, wherein the bar 16a is provided directly on the end of the tendon 11a. As shown, the bar 16a and tendon 11a are integrally formed as a single rod, although it will be appreciated that the bar 16a can also be secured to other types of tendons. The coupler 17a, as shown in FIGURE 8, is again secured to the tendon 12, and preferably comprises a double-ended coupler of the type shown in FIGURE 5. Separate jack engaging means 19a and 21a are here provided as protuberances on the tendons 11a and 12a, independent of the shoulders of the coupler 17a. Preferably, the protuberances are formed as removable wedge grip nuts, similar to the nuts 58 and 61 aforementioned, to be described more fully. To connect the tendons 11a and 12a, the jacking means force the two tendons together and into engagement with the two ends of the coupler 17a, whereupon the wedges of the coupler grippingly engage the tendons and hold them against return movement. As described with the embodiment in FIGURE 2, when the force of the jacking means is released, the setting of the wedge cam planes causes the initial tension in the tendons to be decreased. However, again in this embodiment, it is possible to take up the slack caused by this setting by applying force again for a second time thereby placing the ends of the tendons intermediate the wedge nuts in a slack unstressed condition. By providing the two Wedges of the coupler 17a with oppositely wound spiral threads, rotation of the coupler causes the ends of the tendons to be drawn together tightly. Then when the jacking force is released, the tendons will remain in substantially the desired tension condition, as described in the abovementioned application S.N. 302,628.

Wedge grip nuts 19a, 21a as shown in connection with FIGURE 6 and wedge grip nuts 58 and 61 of FIGURE 2 are particularly constructed for mounting either permanently or temporarily upon a bar or rod and for this purpose are split longitudinally for removal therefrom. The special structure of one of these grip nuts is set forth in detail in application S.N. 302,628 and therefore only brief discussion thereof is warranted here. The wedging mechanism of grip nuts 19a, 21a, 58, and 61 provides a one-way gripping action such as that shown for example in FIG- URE 2. Nut 58 is arranged on bar 55 to secure the fluid cylinder from sliding off end 59, while grip nut 61 is arranged to provide a similar service at end 60 of bar 55.

In a similar manner, grip nuts 19a and 21a as shown in FIGURE 8 are disposed on tendons 11a and 12a such that the one-way gripping action of the nuts provides a pair of jack engaging means at shoulders 47 and 48 for receiving an inwardly directed force from the jacking means. This inwardly directed force is transferred to tendons 19a and 21a drawing the opposing ends of same towards one another.

In FIGURE 1, the present invention, as embodied in the above described methods and apparatus, is generally depicted in the operation of tensioning a plurality of rods 106 around a circular tank 107. A bracket 108 is provided on the arm 41 of the jacking means to enable the jack to be supported by a cable (not shown) from a suitable crane or the like disposed on top of the tank 107 and adapted to move along the tanks periphery. In mounting the rods 106 around the tank, a plurality of rods may be simply spliced together at adjacent ends by a coupler 28, as shown in FIGURE 5. This generally is best accomplished by first mounting the wedges separately on the rod ends, after which the housing is threaded onto the pairs of opposed wedges. These spliced rods are then disposed around the tank with the spaced opposed ends of two rods remaining to be joined. As shown, a pair of couplers 17 and 26 and a connecting bar 16 are used to join the remaining ends in the manner described hereinabove with regard to the embodiment of FIGURE 2, thus placing each of the encircling rod assemblies under tension. A high pressure fluid compressor 109 is connected to the fluid pressure fittings 64 and 65 of the hydraulic actuator 49 by a pair of conduits 111 and 112. The compressor 109 is preferably portable for movement around the tank together with the jacking means. The compressor illustrated is one manufactured by Lincoln Engineering Company of St. Louis, Mo., and ordinarily sold as a high pressure lubricant pump. This unit is airmotor operated and may be produced to provide a 70 to 1 ratio so that pounds per square inch air pressure will yield 7,000 pounds per square inch oil pressure for application to the hydraulic actuator 39. Of course, any suitable commercially available high pressure hydraulic source may be used.

It will be apreciated that with a relatively large tank, the joining together of the couplers 17 and 26 may not provide the desired tension in the tendons around the whole tank, since nonuniformity of tension is likely to be caused by friction. Consequently, it may be desirable to draw each pair of opposed tendon ends together to tension them, rather than simply splice them as shown, in addition to applying tension to the last pair of ends to be connected. To do this, considering the embodiment of FIGURE 6, for example, the wedge members of the coupler would be separately mounted on the tendon ends, apart from the coupler housing, but just far enough to hold them on during initial tensioning. Then the coupler housing is threaded onto the wedges in the manner described to form the splice. By mounting the split wedge nuts on each tendon end, as shown in FIGURE 6, it is then possible to bring the jack to each splice joint and draw the tendons all the way into the wedges, thus taking up slack at each splice position. After all the splices are tightened in this manner, further removal of slack can be accomplished, as described above, with regard to the second application of force at each splice connection, and rotation of the coupler to draw up the tendon ends as tightly as possible. Each of the splice connections, of course, can similarly be made in accord with the embodiments of the invention depicted in FIGURES 2 and 6. In order to measure the tension in the bars to assure that the desired stressing conditions are achieved, it is preferable that a device such as a Howlett .Extensometer be used in conjunction with each slack take-up point to observe the tensile forces as the jack is operated. Preferably, the housing of the present coupler is formed of high strength steel so as to carry the loads imposed upon it and the wedge members are preferably formed of hardened steel so as to take a proper bite into the rods.

From the foregoing it is apparent that the present invention affords a highly convenient method and apparatus for connecting and placing tendons under tension, and which can be utilized to utmost advantage in providing extremely high tensile forces that aproach the maximum tensile strength of the tensioned members.

I claim:

1. In a concrete prestressing apparatus for drawing to- I on said link and adapted for movement in substantially I the same plane, said arms each having a jaw assembly including a bifurcated jaw portion with two spaced elements adapted to receive one of said tendons therethrough, said assemblies being disposed in spaced confronting relation-.. ship and each including a pivotally mounted jaw grip I having a slotted opening adapted to receive one of said tendons therethrough, said jaw grips being disposedin spaced confronting'relationship and eachybeing engageable respectively wi'th jack engaging means on one of said tendons, and means for pivoting said arms to move said jaw portions from and towards each other.

2. The jack means characterized in claim 1 further defined by said jaw grip and'its corresponding jaw portion having complementary arcuate bearing surfaces, and wherein said jack means include an elongated bar pivotally secured to one of said arms intermediate the jaw portion thereof and said link, said bar being disposed in generally parallel relationship with said link and being mo'vable axially relative to said other arm, and a fluid pressure cylinder member having a piston member slidably disposed therein with one of said members being secured to said bar and the other of said members being pivotally mounted on said other arm whereby relative movement of said piston and cylinder members moves said arms from and towards one another.

3. The jack means of claim 1 further characterized by said jack means including a generally elongated link, a

pair of arms pivotally mounted at spaced positions on said link and adapted for movement insubstantially the same plane, said arms each having a bifurcated jaw portion withtwo spaced elements adapted to receive one of said tendons therethrough, said jaw portions being disposed in spaced confronting relationship and each being engageable respectively with jack engaging means disposed on one of said tendons, means for pivoting said arms to move said jaw portions fromand towards each other, and said elongated link including means for selectively adjusting the length thereof whereby said arms can be maintained in i generally parallel relationship for diiferent spacings of said jaw portions.

References Cited UNITED STATES PATENTS 1,929,026 10/1933 Marcil 254-78 FOREIGN PATENTS 178,381 7/1917 Canada.

OTHELL M. SIMPSON, Primary Examiner. 

